Connecting retention gap...

[Tommy Krak]

Good day All,

I have tried so many times, with little success to connect retention gap to analytical columns using glass unions.

Here is what I do: I cut the column squarely (in my opinion), soften the polyimide used methanol and place the columns in the glass union and hold it for brief period of time. They come apart on me, a slight air leak develops and my peak shape for my most active analytes is destroyed.

Has anyone any useful tips on how to perform this task?

Regards,

Tom

[Bigbear]

I have had no luck with the glass press fit connectors. In the past I used a Supelco product I can't remember what it was called. It was a SS body with a double tapered ferrule . It worked quite well ( after assembly , was difficult until you got the hang of it)!
Some folks like a Restek version of what you are using. It has a cage to support the union and nuts and ferrules to make the connections.

I also remember some folks using some sealing compound ( poly amide?) with the connections you use.

Oh yeah some manufacturers build columns with included retention gaps.

[Tommy Krak]

Thank you very kindly for your response. The nature of my work suggests I use glass connectors, I have tried a few of those things which seem to develop leaks when the GC oven is cycled (which I believe to be a common occurence). I have some very active analytes which begin to tail with active connectors or small leaks. I have looked at previous responses to my old queries and I shall try submerging the column ends, something I have not yet tried.

Regards,

Tom

[chromatographer1]

Usually the problem is with the thickness of the polyimide coating of the column, which depends upon the manufacturer.

The best coating for connecting the glass unions I have found in the past was from Quadrex Corp. but all makes can be made to work.

Polyimide glue can be helpful, but requires some technique. Not everyone can use it well.

After cleaning the last few inches of column end, make a nice sharp 90° cut and soak the column in acetone or methanol for a 10-20 seconds. Reduce the headpressure of the column to 1-2 psi instead of the 6-20 psi usually set. Press continuously and firmly the column into the union and HOLD IN PLACE UNDER PRESSURE so the polyimide coating will press into place (stick in place). Heat the connection isothermally in the oven for a few minutes (@ 100 - 200°C), then open the oven and press the column again into the union firmly and continuously for several seconds. Examine the continuous ring of the seal to make certain there are no breaks in the seal. Now restore the pressure of the column to its operating parameter.

I rarely had leaks as long as I carefully wiped the last few inches of the column clean before I cut the column. I also checked the seal before heating the union to high temperatures.

I have used the FS coated unions from Restek for connecting different OD FSOT tubing quite successfully. If you are connecting the SAME OD FSOT columns/tubing then a bore through union FS coated using the proper ID union will also work well if the ends are cut flat and a good installation is made.

best wishes,

Rod

[Tommy Krak]

Thank you very kindly for your response Rod. I had never considered cleaning the column before making the cut. I have just installed a new union doing something a little different than I had before, that being, I soaked the column ends in methanol as opposed holding a methanol soaked Kimwipe on the columns. Next time I shall clean the column ends before making the cut! These unions are making me crazy.

Regards,

Tom

[chromatographer1]

Tom,

Remember:

I also checked the seal before heating the union to high temperatures.

Let me add, don't put stress on the column connection (pulling tension, usually from weight of the column cage)

This is very important as well.

best wishes,

Rod

[Tommy Krak]

Rod,

Thank you very kindly once again. I check for leaks with both: an electronic leak detector; and I am also using HR-MS so as an indirect measure of oxygen levels I look for argon.

Once my source is heated up even before I begin to heat the GC oven I have a look at my argon levels.

I can sporadically do this glass union thing, though I have tried other things like Agilent's Ultimate Union which seemed to work well until I started my most difficult analytes, the lowest standard was not looking so good. I now realize that the Siltek glass unions give me the best results.

Thank you very much once again for your guidance!

Cheers,

Tom

[Peter Apps]

I use Restek siltek Y connectors two or three at a time in flow switching inlets and Deans switches. The most critical step is a clean right-angled cut on the column - inspect it with a x10 magnifier and if it is not completely square and clean do it again. Practising on an old bit of column is time well spent. I wipe the end of the column and silica transfer lines with methanol, but I do not soak it. Then I push the end into the connector - how hard to push is difficult to describe, about as hard a writing with an HB pencil maybe. If you can get at the connector to inspect the join you should see a uniform ring of dark brown where the polyimide is sealed to the inside of the connector. Push too hard and the end of the column will crack. Once you get the feel for it, it becomes pretty much routine.

Peter

[Tommy Krak]

Thank you very kindly Peter. I have been practicing on some column quit a bit. I am getting the hang of it. However the tips and suggestions that have been given are much appreciated.

Cheers,

Tom

[chhubert]

I previously used fused silica pressfit connector quite often. Don't get the borosilicate one which the heat expansion coefficient may be different than that of the fused silica. If borosilicate is used with fused silica capillary and heated in the oven, the polyimide seal may tear off due to different expansion of fused silica and borosilicate. Also I found that old column is more difficult to get good seal 'cos the polyimide coating is already hardened. In that case, I would apply a thin film of polyimide glue on the capillary column. Allow the glue to harden a bit by holding it for, say half an hour. Then square cut that part of the capillary with glue and make the connection. Of course, as Peter says, practice is very important.
Hope this helps.
Hubert

[Tommy Krak]

Thank you as well Hubert. Yeah, I have to get a handle on this the new connection I made last night just let go. I shall try again on Monday.

Cheers,

Tom

[chemstation]

I remember a long time ago in a previous employ using the glass connectors, but I seem to recall using Hexane.

For leak detection, after I had pressed the column into the pressing, and saw the nice brown ring form, as mentioned in peters post, I then allowed the hexane to evaporate, and then I would fill up a watch glass with hexane, and submerge the connector in it, turn on the inlet pressure and look for bubbles, then increase the pressure to the pulsed split-less pressure, and make sure it still did not leak. As I found electronic leak detectors too insensitive.

I too found that this/any connectors were a major source of active sites, causing my analytes to disappear,
so I ended up connecting the column directly into the inlet, and trimming the front of the column, and then discarding the column when it got too short, may not appear to save money for the company, but it saves your sanity, as well as stopping the need to repeat work which I am of the opinion that repeating work costs more than the regular replacement of a column and improves quality and reliability of results.

In my current employ, they inject into a newly installed liner/column several times with a BSTFA/ACN soln to de-activate any active sites, I also suspect this may also de-activate any active sites on the column too.

Alex

[chromatographer1]

The problem with these is using too high a temperature and cycling a wide range of temperatures. The polyimide will flow and then eventually lose its seal. This is my guess why Tom is having such a problem with them.

This is also why using graphite ferrules and fused silica deactivated metal unions are so useful, and with care so durable. It is also why some who insist on using glass unions use the polyimide glue (available from Supelco and GRACE-Davison, formerly Alltech) to permanently seal the FSOT tubing inside the glass union.

Rod

[Peter Apps]

The problem with these is using too high a temperature and cycling a wide range of temperatures. The polyimide will flow and then eventually lose its seal. This is my guess why Tom is having such a problem with them.


Rod

Temperature cycles might well be the problem - my switch set ups are in their own heated zone that stays hot all the time.

Peter

[Tommy Krak]

Good day All,

Thank you all very much for your guidance. I have a lot of ideas I can work with here. Alex you certainly are right these glass unions are costing me my sanity.

Cheers to All,

Tom